What Is a Public Sewer?
A public sewer is a drainage pipe that is adopted (owned and maintained) by the water company — in London, Thames Water. Since 2011, many private sewers (those shared between properties) were transferred to water company ownership, significantly expanding the extent of the public sewer network.
If a public sewer crosses your garden, any extension, driveway, outbuilding, or other structure built over or near it requires consent from Thames Water.
How to Check for Sewers on Your Property
The most reliable method is a Thames Water drainage search, which can be obtained as part of a property search during conveyancing or independently via the Thames Water website.
Thames Water also provides an online map (the Asset Map) that shows the approximate location of public sewers. Note that this is indicative only — a formal drainage search provides the legally reliable record.
When Is Build-Over Consent Required?
Thames Water requires a build-over agreement when you propose to:
- Build over a public sewer
- Build within 3 metres of a public sewer with a depth up to 3 metres
- Build within 6 metres of a public sewer at depth greater than 3 metres
The consent process is required before work starts. Building without consent exposes you to the risk of being required to demolish or alter the structure.
The Build-Over Application Process
Thames Water operates a build-over consent process with two routes:
Self-Certification (Minor Works)
For straightforward extensions with simple concrete strip or pad foundations over sewers of 150mm or less, Thames Water has a self-certification route. The contractor certifies that specified construction conditions have been met. This applies if:
- The sewer is 150mm or smaller in diameter
- The extension footprint over the sewer is limited
- Standard protective conditions are followed (no rigid connection to the structure, minimum cover depth, etc.)
Formal Application
For larger or deeper sewers, or where the extension has a significant bearing on the sewer, a formal build-over application is required. Thames Water will:
- Review structural drawings
- Assess the impact on sewer access and integrity
- May require CCTV survey of the sewer before and after construction
- May require protective measures such as concrete protection slabs or sleeving
The formal process typically takes 4–6 weeks.
Design Implications of a Sewer
Discovering a sewer crossing a proposed extension footprint can affect the design in several ways:
- Foundation type: A raft foundation may be specified instead of strip footings to bridge the sewer without loading it
- Extension position: Moving the extension footprint to avoid the sewer is sometimes the most cost-effective solution
- Access: Thames Water may require a manhole to remain accessible within the extension — this can affect internal layout or floor level
- Drainage connections: Connecting the extension's drainage to the public sewer must be done correctly to avoid cross-connections or unauthorised connections
What Happens If a Sewer Is Damaged?
If construction damages a public sewer, Thames Water can require repair at the developer's cost. This can be expensive and involves excavation, dewatering, and specialist drain repair. CCTV surveys before and after work are standard on formal build-over applications precisely to establish the pre-work condition.
Connecting to the Public Sewer
If the extension includes a new WC, bathroom, or utility room, a new drain will need to connect to the existing drainage. This typically involves:
- Locating the nearest manhole or suitable connection point
- Breaking into the existing drain under a Building Regulations Drainage notification
- Laying new drain runs to the connection point
- Inspection by Building Control
Thames Water approval is required for connections to public sewers within the highway (usually only relevant for new houses or commercial developments) — for most residential extensions, the connection is to the existing private drainage which then connects to the public sewer at an existing manhole.
RCB Design & Build
RCB carries out drainage searches at the pre-construction stage for all extension projects and coordinates build-over consent applications with Thames Water where required. We ensure drainage design is integrated with the structural and architectural drawings from the outset.